A wide variety of IMDs that deliver a therapy to and/or monitor a physiologic condition of a patient have been clinically implanted or proposed for clinical implantation in patients. IMDs may deliver therapy or monitor conditions with respect to a variety of organs, nerves, muscles or tissues of the patients, such as the heart, brain, stomach, spinal cord, pelvic floor or the like. In some cases, IMDs may deliver electrical stimulation therapy via one or more electrodes, which may be included as part of one or more elongated implantable medical leads.
For example, an implantable cardiac device, such as a cardiac pacemaker or implantable cardioverter-defibrillator, provides therapeutic stimulation to the heart by delivering electrical therapy signals such as pulses or shocks for pacing, cardioversion, or defibrillation via electrodes of one or more implantable leads. As another example, a neurostimulator may deliver electrical therapy signals, such as pulses, to a spinal cord, brain, pelvic floor or the like, to alleviate pain or treat symptoms of any of a number of neurological or other diseases, such as epilepsy, gastroparesis, Alzheimer's, depression, obesity, incontinence and the like.
Exposure of the IMD to a disruptive energy field may result in undesirable operation of the IMD. The IMD may be exposed to the disruptive energy field for any of a number of reasons. For example, one or more medical procedures may need to be performed on the patient within whom the IMD is implanted for purposes of diagnostics or therapy. For example, the patient may need to have a magnetic resonance imaging (MRI) scan, computed tomography (CT) scan, cardioversion, electrocautery, diathermy or other medical procedure that produces a magnetic field, electromagnetic field, electric field or other disruptive energy field.
The disruptive energy field may induce energy on one or more of the implantable leads coupled to the IMD or directly on one or more components of the IMD. The IMD may inappropriately detect the induced energy on the leads as physiological signals. Alternatively, or additionally, the induced energy on the leads may result in the inability to correctly detect physiological signals. In either case, detection of the induced energy on the leads as physiological signals may result in the IMD delivering therapy when it is not desired or withholding therapy when it is desired. In other instances, the induced energy on the leads or on the components of the IMD may result in inadvertent stimulation or heating of the tissue and/or nerve site adjacent to the electrodes of the leads or adjacent to the housing of the IMD. Such heating may compromise pacing and sensing thresholds at the site, which could result in reduced therapy efficacy.